<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    p {
      background: #001;
    }

    #box p {
      background: #002;
    }

    #tag {
      background: #0030;
    }

    div #tag {
      background: #004;
    }

    .box p {
      background: #005;
    }

    #box p {
      background: #002;
    }
  </style>
</head>

<body>
  <div id="box" class="box">
    <p id="tag" class="tag">我是谁</p>
  </div>
</body>

</html>